Equilibrium and Dynamics of Polymer-Grafted Nanoparticles at Fluid Interfaces

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1332836PI: TiltonA polymer brush is a descriptively named monolayer of polymer chains, each of which is anchored by one end to a surface, while the rest of the chain stretches away from the surface into a solvent. Prior NSF-supported research by this group developed polymer brush-decorated nanoparticles that were extremely efficient stabilizers of oil-in-water or water-in-oil emulsions. These nanoparticle brushes stabilized emulsions for several months to well over a year without emulsion droplet coalescence or macroscopic oil and water phase separation. Most importantly, they did so while only requiring nanoparticle concentrations that were two orders of magnitude lower than are generally required for conventional particles used to stabilize emulsions. The goal of the proposed research is to determine the fundamental interfacial phenomena responsible for the unique emulsifying efficiency of nanoparticle brushes. Current understanding of particle-stabilized Pickering emulsions mainly concerns the origins of long-term stability, but it cannot yet explain why particles with different physico-chemical properties produce emulsions with quite similar long-term stability yet display vastly different emulsifying efficiencies, that is, why they require vastly different concentrations to produce a stable emulsion in the first place. This requires fundamental investigation of the interfacial properties of the nanoparticles when they adsorb to the oil/water interface. The main hypothesis guiding this research is that those nanoparticle brushes that produce the strongest interfacial tension reduction and interfacial elasticity at the lowest surface concentration will be the most efficient emulsifiers. A series of nanoparticle brushes with well-defined compositions and architectures will be synthesized by controlled radical polymerization. For each nanoparticle brush, the surface equation of state (the relationship between interfacial tension and the adsorbed particle surface concentration), interfacial dilatational elasticity, and diffusional dynamics will be measured at the oil/water interface and correlated with emulsifying efficiency. Nanoparticle brushes are a new class of surface active material. Typically, such materials are designed to manipulate the interface between incompatible materials to make them more compatible. For example, they mix oil and water in emulsions that deliver active ingredients in pharmaceuticals or personal care products. This research is transformative for its focus on the fundamental reasons why nanoparticle brushes are extremely efficient emulsifiers and for the broader fundamental understanding it will provide for how chemical structure controls the interfacial behavior of this new class of surface active materials. This knowledge will enable high stability emulsion products with ultra-low emulsifier concentrations. Besides the benefit of efficient raw materials usage, this also promises new products that are infeasible without emulsifiers that are both highly effective and highly efficient. One possibility is a clean-burning diesel fuel emulsion that meets stringent stability requirements without excessive additives. Broader impact for technology workforce development is delivered by the interdisciplinary research education of Chemical Engineering and Chemistry Ph.D. and undergraduate students. Pittsburgh middle school students from under-represented groups will be mentored in closely-related, discovery-based science projects that teach students how they can use scientific understanding to make predictions ? in this case concerning interfacial tension reduction and emulsification performance of different kinds of materials. Seminar visits with high school chemistry students will introduce the full spectrum of chemistry-related careers to encourage retention in the STEM pathway as they transition to college.This reasearch is jointly funded by the Particulate and Multiphase Processes and Interfacial Processes and Thermodynamics Programs in the CBET (Chemical, Biological, Environmental and Transport) division in Engineering.